1. Field of the Invention
The present invention relates to interferometers for making highly accurate measurements of wavefront aberrations, particularly to phase-shifting point diffraction interferometers.
2. State of the Art
Optical metrology is the study of optical measurements. An area of optical metrology relevant to the present invention is the use of an interferometer to measure the quality of a test optic, such as a mirror or a lens.
One important recent application of optical metrology is the testing of projection optics for photolithography systems. Modern photolithography systems used to fabricate integrated circuits must continually image smaller features. To do so, systems are confronted with the diffraction limit of the light employed to image a pattern provided in a reticle. To meet this challenge, photolithographic systems must employ successively shorter wavelengths. Over the history of integrated circuit fabrication technology, photolithography systems have moved from visible to ultraviolet and will eventually move to even shorter wavelengths, such as extreme ultraviolet.
Because of the increasing difficulties posed by directly imaging a reticle pattern onto a wafer, it is desirable to use projection optics in lithography systems. Such systems include lenses or other optical elements that reduce the reticle images and project them onto the wafer surface. This allows reticles to retain larger feature sizes, thus reducing the expense of generating the reticle itself.
As with all optical imaging systems, various aberrations such as spherical aberration, astigmatism, and coma may be present. These aberrations must be identified and removed during the fabrication and/or alignment of the projection optics, or the projection optics will introduce substantial blurring in the image projected onto the wafer.
In order to test the projection optics for various aberrations, interferometers may be employed. Conventional interferometers, based upon the Michelson design, for example, employ a single coherent light source (at an object plane) which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram and resultant wavefront with, for example, Zernike polynomials, indicates the presence of aberrations.
The reference wave of the interferometer should be "perfect"; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optical elements through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always contains some artifacts from the optical elements through which the reference wave passes. While these artifacts, in theory, can be separated from the interferogram, it is usually impossible to know that a subtraction produces a truly "clean" interferogram.
To address this problem, "point diffraction interferometers" have been developed. An example of a point diffraction interferometer is the phase-shifting point diffraction interferometer described in the article H. Medecki, "Phase-Shifting Point Diffraction Interferometer", Optics Letters, 21(19), 1526-28 (1996), and in the U.S. patent application "Phase-Shifting Point Diffraction Interferometer", Inventor Hector Medecki, Ser. No. 08/808,081, filed Feb. 29, 1997 now U.S. Pat. No. 5,835,217, which are both incorporated herein by reference. Referring to FIG. 1, in this prior art phase-shifting point diffraction interferometer, electromagnetic radiation is sent to a pinhole 22. The radiation is then sent through the test optic 24 to a grating 26. Equivalently, the order of the grating and the test optic may be reversed. The grating 26 produces two beams with a small angular separation. An opaque mask, placed near the focal point of the test optic, contains a tiny reference pinhole, and a larger window centered on the respective foci of the two beams. The reference pinhole produces a reference wavefront by diffraction, while the window transmits the test wave without significant spatial filtering or attenuation. In effect, the beam going through the reference pinhole is filtered to remove the aberrations imparted by the test optic thereby producing a clean reference wave. The two beams propagate to a mixing plane where they partially overlap to create an interference pattern recorded on a detector 30. The light in the interferometer will typically be of a single wavelength. The grating 26 will transmit the zeroth- order beam straight through, but will produce a small angular change to the first-order diffractions. In the image plane 28, the zeroth-order, and the first-order diffractions will be in different positions, as indicated by the reference pinhole and the test window in the mask 28. The zeroth-order goes to the test beam window and the first-order goes to the reference pinhole. Phase-shifting is provided by translating the grating 26 perpendicular to the rulings of the grating. Phase-shifting improves the accuracy of the system.
The phase-shifting point diffraction interferometer tends to suffer from relatively low fringe contrast which makes the signal more susceptible to noise and therefore has the potential of limiting the accuracy of the interferometry. This low contrast is due to the imbalance between the zeroth-order test beam and the first-order reference beam and the imbalance is further aggravated by the spatial filtering of the reference beam. As is apparent, there is a need for improving the fringe contrast and thus the signal-to-noise ratio.
Previous endeavors to achieve test beam balance include, for example, increasing the size of the phase-shifting point diffraction interferometer reference pinhole. This method is not acceptable because the accuracy of the phase-shifting point diffraction interferometer improves as the reference pinhole gets smaller. An alternative method for balancing the beams involves placing an attenuating membrane in the test-beam window. This method is also not acceptable because of membrane damage and contamination caused by extreme ultraviolet radiation reduces the accuracy of the phase-shifting point diffraction interferometer.